Generally, optical recording media have been widely used because of their excellent properties such as high recording capacity, capability of non-contact recording and playing-back, and the like. In rewritable optical discs such as WORM, CD-R, DVD-R, and the like, laser beam is collimated onto a quite small area in an optical recording layer to perform recording through changing the properties of the optical recording layer at the area, and playing-back is performed based on the difference between the amount of light reflected in the recorded area and that in an unrecorded area.
Currently, in the above-mentioned optical disc, the wavelength of semiconductor laser used for recording and playing-back is 750 to 830 nm for CD-R, or 620 to 690 nm for DVD-R. However, for further increasing the capacity, there is examined an optical disc to which a laser having a shorter wavelength is applied; for example an optical disc to which light in 380 to 420 nm is applied for recording.
For an optical recording layer in such an optical recording medium for short-wavelength light, an indole derivative has been examined as an optical recording material. As the indole derivative, for example, Patent Document 1 reported a monomethinecyanine compound, and Patent Document 2 reported an indole compound.
For the optical recording material, the wavelength of the absorption maximum (λmax) is required to be suitable for the light for recording and the light for playing-back. Furthermore, an optical recording material having a high absorbance is advantageous in terms of recording sensitivity and recording speed. The above-mentioned compounds did not necessarily have characteristics of the absorption wavelength suitable for short-wavelength lasers.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-237233
Patent Document 2: WO 01/44374 Pamphlet